MCM3 Safeguards Neural Progenitor Maintenance and Cortical Development Against Replication-Associated Stress.

Faithful genome duplication during neurogenesis relies on the licensing of surplus replication origins, and hypomorphic variants in MCM3 have been associated with microcephaly and related growth disorders. Using in utero electroporation in embryonic mouse cortex, we show that acute partial depletion of MCM3 disrupts cortical progenitor development. MCM3 knockdown reduced EdU incorporation and PCNA positivity among GFP-positive cells in the VZ, indicating decreased S-phase engagement or impaired cell-cycle progression. It also increased replication-stress-associated signals, including γ-H2AX and p-RPA2(T21), but did not induce detectable cleaved caspase-3 activation. These changes were accompanied by a reduction in the population of electroporated neural progenitors, leading to decreased neuronal output. In addition, MCM3-depleted neuronal progeny showed altered radial distribution and reduced callosal axon extension. Together, these findings support a role for MCM3 in maintaining progenitor proliferative capacity and genome-stress tolerance during cortical development, and provide mechanistic insight into how partial MCM3 deficiency may contribute to microcephaly-related neurodevelopmental disorders.